Currently most high-selectivity gas analyzers are based on correlational spectroscopy, wherein gas concentrations are determined from characteristic features of the spectra of gases under analysis, such, e.g., as the quasiperiodic structures of absorption or transmission spectra. These gas analyzers should be capable to perform multicomponent analysis of the gas under study without essentially complicating their design and also feature simplicity of conversion to measurement of the content of another gas in the mixture.
Known in the art is a correlational gas analyzer ("Prikladnaya infrakrasnaya spektroskopiya", Ed. D. Kendall, Moscow, MIR Publishers, 1970) comprising a light source and positioned in sequence along the beam path interference filter to select the specified spectral band of the gas under analysis, a modulator, two cells with one cell thereof filled by the gas under analysis and the second cell thereof filled with a gas that does not absorb radiation in the selected spectral band, a photoreceiver, and a recorder.
After passing through the cell filled with the gas under analysis the light beam does not contain spectral components corresponding to absorption lines, while at the output of the other cell it contains all spectral components of the selected for gas analysis spectral band. The photoreceiver generates a signal proportional to light attenuation due to absorption in the gas under analysis and this signal allows detection and assessment of concentration of the gas under analysis positioned in the beam path between the light source and photoreceiver.
The known in the art gas analyzer features poor accuracy and low reproducibility of measurement results due to absorption of the gas under analysis and its leakage out of the cell. In cases of corrosive gases, such as H.sub.2 S and SO.sub.2, and unstable gases, such as NO.sub.2, using this gas analyzer is hampered by the necessity to maintain constant temperature and humidity.
Also known in the art is a correlational gas analyzer (R. Haulet, C. Vavasseur "Teledetection des pollutants gaseoux de l'atmosphere" Bull. inform. sci. et techn., 1978, 230/231, p. 59) comprising a light source, the beam whereof passes through the gas under study with a quasiperiodic spectrum pattern in the specified spectral band, and through an optical system comprising sequentially positioned along the beam path condensor, input slit iris, dispersing element, and output slit iris mounted rotatably and configured as a disc with a slit to scan the specified spectral band arriving at the photoreceiver, with the output thereof electrically connected to the input of a recorder via an electric signal amplifier.
This known in the art correlational gas analyzer uses a concave grating as the dispersing element, while the slits in the disc are configured as arcs and positioned so as to coincide with maxima and minima in the absorption spectrum of the gas under study in the focal plane of the concave grating.
The light beam passed through the gas under analysis is decomposed into a spectrum by the concave grating and then passed via the slits of the rotating disc. Thus the spectral band of the gas under study is scanned by the photoreceiver, the modulation depth of the light beam being proportional to difference in intensities of corresponding transmission and absorption areas in the spectral band of the gas under study and depends, therefore, on the content of the gas under study in the volume.
The optical system of this known in the art gas analyzer is complicated and therefore hard to manufacture, as is the alignment of the disc, the slits whereof have to be precisely aligned to the maxima and minima of the absorption spectrum of the gas under study, this alignment being critical to the measurement accuracy and reproducibility. Conversion to measurements of another gas component requires replacement of the disc with another, featuring corresponding slits, and its alignment in the optical system.